(1) Field of the Invention
The present invention relates to an improved membrane switch assembly. Specifically, the present invention relates to a membrane switch assembly having internal cavities that are edge vented to the external environment and methods of manufacture thereof.
(2) Description of the Prior Art
Prior art membrane switch assemblies are usually constructed by laminating a spacer sheet between two substrates which support "printed" circuits. The substrate bearing the printed circuits are positioned so that circuit patterns thereon face each other. The switches are defined by providing the spacer sheet with apertures so that, with the application of pressure, the circuits can be made to contact each other. These prior art membrane switch assemblies were usually constructed so that the apertures within the spacer sheet formed cavities that were permanently sealed from the surrounding environment. These cavities were usually filled with a fluid, primarily air.
The above-described prior art method of constructing membrane switch assemblies has certain disadvantages. A major disadvantage which results from permanently sealing the cavities defined by the spacer sheet apetures occurs when there is a change in the external fluid pressure, the atmospheric pressure for example. If a machine which incorporates the membrane switch assembly is located at an altitude where the outside atmospheric pressure is less than the pressure in the sealed cavities, the greater internal pressure exerts an outward force upon the layers of the switch laminate. The result of this outward expansion is that there is a cushioning effect to the operation of the individual keys. With a sufficiently large pressure differential it becomes difficult for the operator to determine whether the key has been activated. In the extreme situation, when the difference between the outside atmospheric pressure and the pressure within the cavities is quite large, the membrane switch assembly may become distorted with structural damage possibly being caused by the increasing pressure on the laminate walls caused by the outward expansion.
A similar result occurs when the outside atmospheric pressure becomes greater than the atmospheric pressure within the cavities. This would occur, for example, when the mechanism incorporating the membrane switch assembly is operated at a lower altitude than where the laminate was constructed. The result would be that the force exerted upon the wall of the laminate by the outside atmospheric pressure would move the walls of the laminate inwardly. The usual effect of this pressure differential would not be as significant as when the atmospheric pressure is less than the internal pressure. However, in the extreme condition when the pressure differential between the outside atmospheric pressure and the internal pressure becomes great, a switch could be activated.
It is to be observed that even under normal operating conditions, the gas within the cavities resists compression of the walls of the laminate when a user tries to activate the keys. This results in a cushioning effect which is felt by the user of prior art membrane switch assemblies. While under certain circumstances a cushioning effect may be desirable, it may also reduce the user's ability to activate a key or detect whether a key has been actuated.
Several methods have been utilized to try to alleviate the above-discussed disadvantages of prior art membrane switch assemblies. One proposed prior method involves incorporating internal passages, within the laminate between the cavities. This allows displacement of the fluid medium, particularly air, between the internal cavities of the membrane switch assembly. When one key is activated the fluid within the spacer sheet defined cavity associated with that key which is displaced by the downward force of the membrane wall will flow through the passages into one or more other cavities. While this will help to minimize the cushioning effect caused by the resistance of the internal pressure to the downward compression of the membrane wall, it will not alleviate the problems associated with an internal-external pressure differential.
It has also been proposed to equalize the internal pressure with the external atmospheric pressure by providing a hole in the outer layers of the membrane switch assembly commonly referred to as a through-hole. This through-hole in the laminate of the switch assembly allows air to flow freely into and out of the assemblies cavities. While this technique would solve the problems associated with the pressure differential between the external and internal pressures, it creates some of its own disadvantages. The major of these disadvantes becomes apparent with the incorporation of the completed membrane switch assembly into a final product. The through-hole vents would typically be provided through the entire assembly. While the holes on the front surface of the assembly may be sealed off, for example by an indicia bearing sheet, the holes at the back surface must remain clear. This causes difficulties when installing the switch laminate in products such as calculators, microwave ovens, thermostatic controls, etc. The membrane switch assembly would, to keep the through-hole open, have to either be spatially separated from the surrounding housing or the surrounding housing would have to be provided with corresponding holes to allow for a free flow of air into and out of the through-holes. This requires additional manufacturing steps or a larger housing to provide the spatial separation. Furthermore, since many membrane switch assemblies are secured within the final product through use of adhesives, during manufacturing special care would be required to avoid having the adhesive flow into or seal off the through-hole vents.